The present invention relates to ceramic honeycombs for heat regenerator applications. More particularly, the invention relates to ceramic honeycombs which can be manufactured by the extrusion of batches comprising thermally crystallizable glass powders in combination with selected inorganic additives. The resulting honeycombs exhibit high strength and stability under severe thermal cycling such as encountered in combustion engine heat exchanger environments.
A number of processes for the manufacture of cellular ceramic honeycomb bodies have been developed to the point of commercial application. U.S. Pat. No. 3,885,977, for example, describes the production by extrusion of ceramic honeycomb bodies composed of cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) ceramic material. These honeycombs offer high strength and low thermal expansion, and have accordingly been widely employed as durable substrates for automotive exhaust treatment catalysts.
Ceramic honeycombs have also been produced by laying up sheets of ceramic precursor material into a cellular structure. Thus U.S. Pat. No. 3,112,184 describes the fabrication of ceramic honeycombs for heat exchanger (also called heat regenerator or thermal regenerator) applications wherein flat and corrugated sheets of ceramic material are layered to provide a green ceramic assembly which can be fired to produce a ceramic honeycomb of high cell density.
As noted in U.S. Pat. No. 3,600,204, a preferred ceramic material for heat regenerator applications is a lithium aluminosilicate (LAS) glass powder which, upon firing, is converted by thermal crystallization to a low-expansion ceramic material. Such ceramics, sometimes termed glass-ceramics because they originate from glasses, comprise beta-spodumene or a beta-spodumene solid solution (beta-spodumene s.s.) as the principal crystal phase.
LAS powders and low-expansion beta-spodumene glass ceramics produced therefrom offer a number of advantages for heat exchanger applications, such as ceramic turbine regenerators, wherein thermal cycling of the ceramic is frequent and severe. However, the cost of fabricating such regenerators by lay-up processes is still too high. Therefore, it has been proposed to produce similar products by the extrusion of high-cell-density ceramic honeycombs.
The extrusion of glass powders, however, presents several problems. Among these problems are the relatively low packing density of the glass powders, resulting in a need for higher levels of organic binders in glass-powder-based extrusion batches in order to successfully extrude complex shapes. As a consequence, attempts to produce products by the extrusion of glass powder batches have not been successful, due to the very high firing shrinkages incurred.
High firing shrinkages would present particularly difficult problems in the production of thin-walled honeycomb structures by extrusion. Among such problems are a higher risk of distortion of the structure during sintering, and a higher incidence of cracking defects in the product. In addition, larger dies would be required to produce the oversize green honeycombs needed for the final product, and dies for the extrusion of large honeycombs are particularly difficult to fabricate.